The image transfer mechanism in many printing presses is based on hot and pressurized thermal transfer. Such printing presses generally incorporate an intermediate transfer medium (ITM) drum and an impression (IMP) drum between which is defined a nip though which paper or other printing medium is drawn, and onto which paper an image is transferred in known manner. In order to achieve optimal performance in terms of PQ, transferability and durability of the image, the pressure level and uniformity along the nip, and therefore from side to side of the ITM drum, must be strictly controlled. Currently, the dimensions of the nip rather than the pressure along the nip is monitored.
If a strain gauge is incorporated, then an open loop control is achieved on the pressure along the nip. However, such open loop control still requires manual adjustments and suffers from an inability to make the required corrections in high speed machines, especially in cases when the printing is to be carried out on several kinds of paper with different grammage. In such situations, several null cycles are required in the switching of the paper.
European patent application EP 1594017 discloses a fuser system of a xerographic device having a fuser drum and a pressure drum which in use bear against one another such as to form a nip, and which system utilises closed loop feedback to control the nip pressure.
The system of the present invention has been developed to overcome the above mentioned problems. In addition the present invention is adapted to eliminate or reduce printing artefacts e.g. banding which are formed when the blanket drum rolls into the region of the gripper's gap in the impression drum. At the start of the gap the force between the blanket drum and the impression drum goes abruptly to zero causing the printing unit to vibrate and also accelerate the blanket drum. When the drums come into contact again at the end of the gap the pressure abruptly builds up again. These fluctuations of the contact pressure and circumferential speed cause printing artefacts. The printing assembly of the present invention provides a high bandwidth along with utilization of advanced control options such as: feed-forward and gain scheduling in order to allow the printing assembly to adequately withstand the abrupt torque disturbance keeping the IMP drum position and speed.